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Mercury Exposure in a High School Laboratory -- Connecticut

On December 8, 1986, 22 students and a teacher in a Connecticut high school chemistry laboratory were exposed to mercury vapor. The class was conducting an oxidation reduction experiment that called for silver oxide. However, mercuric oxide had been used because silver oxide was not available.

The experiment was performed at eleven work stations; exhaust hoods in the classroom were not turned on. Each experiment used 1.75 g of mercuric oxide to obtain a theoretical yield of 1.62 g of elemental mercury. The mercuric oxide was placed in a crucible and heated over a burner flame for 15 minutes to drive off the oxygen. The teacher stopped the experiment when he learned that the yields were lower than expected, and, therefore, mercury was being vaporized. He turned on the hoods and had the students clean out the crucibles. The experiment had started at approximately 8:15 a.m.; the students had left the room by 9:00 a.m. The school then called the local fire department and the Toxic Hazards Section of the Connecticut Department of Health Services for assistance in determining the extent of the possible mercury exposure.

The maximum concentration of mercury in the air was estimated at 50 mg/m3 (10.9 g total mercury lost divided by 219 m3 air volume of room).* The mercury saturation point in air at 20 degrees C (68 degrees F) is 15 mg/m3 (1). The excess 35 mg/m3 of mercury that appears to have been lost may have condensed on surfaces in the room. The maximum dose, or body burden, to each student was estimated at 9.3 mg.**

Air measurements for mercury were taken in the laboratory after it had been ventilated for several hours. The mercury level was 0.008 mg/m3 with the windows open and hoods on. However, when the laboratory was closed and the hoods were turned off for 25 minutes, the level rose to 0.04 mg/m3 (the American Conference of Government Industrial Hygienists time-weighted average is 0.05 mg/m3). This fivefold increase may have been due to vaporization of the condensed mercury from surfaces in the room. Mercury levels were measured again the day after the incident (December 9), and school personnel were given instructions for cleanup. On December 12, mercury levels in the air in the room ranged from 0.002 to 0.003 mg/m3. School officials were told they could resume use of the classroom.

On December 11, urine samples were obtained from the 23 persons who were in the classroom during the experiment. Eight persons had urine levels of mercury at or above 30 ug/L, the maximum level considered acceptable (2). On January 20, 1987, repeat tests showed that six of the eight students still had urine mercury levels above 30 ug/L. School officials decided to have follow-up testing performed on the remaining 15 persons in the class. The urine mercury level for all but one of these 15 persons had increased from the original value, and some had risen to 30 ug/L or above. The highest level was 72 ug/L. Testing of a control group to determine the normal average urine mercury level for unexposed students at the school was also requested. However, school officials did not allow control samples to be obtained. Additional follow-up testing was conducted on February 24, 1987, and again on March 31, 1987. On February 24, 1987, everyone in the class, including the teacher, had a mercury level either at or below 30 ug/L. On March 31, 1987, one student had a mercury level of 37 ug/L; all others remained at or below 30 ug/L. Reported by: M Shelnitz, H Rao, PhD, CJ Dupuy, MS, SM, B Toal, MSPH, M Cartter, MD, JL Hadler, MD, MPH, State Epidemiologist, State of Connecticut Dept of Health Svcs. Div of Environmental Hazards and Health Effects, Center for Environmental Health and Injury Control, CDC.

Editorial Note

Editorial Note: The biologic half-life for mercury vapor ranges from 35 to 90 days (3). Immediately after exposure, fecal excretion of mercury is predominant; renal excretion increases with time (3). Careful behavioral and neurological monitoring is recommended when urine levels are 100 ug/L or greater (4). Seventy-eight days passed between the students' exposure on December 8, 1986, and the test on February 24, 1987, in which all urine mercury levels were at or below 30 ug/L. The fact that one to two biologic half-lives had passed during this time probably explains the decrease in urine mercury concentrations.

Organic mercury, which is predominantly methyl mercury, and elemental mercury pose different risks. These differences result from the greater intake of organic mercury, which is obtained through the diet, and from the intrinsic toxicities of both forms of mercury (5). High doses of methyl mercury can produce irreversible destruction of neurons in the visual cortex and cerebellum and lead to a permanent narrowing of the visual field and signs of ataxia (5). The effects of inhaled mercury vapor on the nervous system are usually reversible, particularly if they are mild (5).

Much of the information on elemental mercury vapor is qualitative rather than quantitative, but good quantitative dose-response data are available for methyl mercury. Since methylated mercury poses greater risk than vaporized mercury, it was considered feasible to use these data in analyzing the possible risk of adverse effects in the Connecticut incident. Methyl mercury exposure has been shown to cause neurological effects at body-burden levels of between 25 and 50 mg (3). The students' estimated body burden of 9.3 mg was well below these values; therefore, neurotoxic effects were not anticipated. Acute renal effects were not anticipated either because they are generally caused by inorganic mercury salts (3).

The appropriate method for determining risks associated with toxic chemical exposures is to measure and compare ambient concentrations and body burdens. Such analysis allows for the examination of factors that can affect absorption at different exposure levels. However, as in the incident reported here, such data are not always available. In the absence of good monitoring data, estimated body burden must be used to assess risk.

The problem that occurred at this high school could occur at other schools. Consequently, it is recommended that mercuric oxide not be substituted for silver oxide. In the event of mercury exposure, workers assigned to cleanup should be warned of the danger involved and instructed in safety precautions. Also, students should be trained in the proper use of laboratory safety equipment such as exhaust hoods, goggles, gloves, aprons, and fire extinguishers as well as in the proper disposal of toxic chemicals that are used in classroom experiments.

References

  1. Dreisbach RH. Handbook of poisoning: prevention, diagnosis, & treatment. 10th ed. Los Altos, California: Lange Medical Publications, 1980:234. 2. Nobel S. Mercury in urine. In: Seligson D, ed. Standard methods of

clinical chemistry. Vol 3. New York: Academic Press, 1969:180. 3. Goyer RA. Toxic effects of metals. In: Doull J, Klaassen CD, Amdur

MO, eds. Casarett and Doull's toxicology: the basic science of poisons. 3rd ed. New York: Macmillan Publishing, 1986:605-9. 4. Adams CR, Ziegler DK, Lin JT. Mercury intoxication simulating

amyotrophic lateral sclerosis. JAMA 1983;250:642-3. 5. Environmental Protection Agency. Mercury health effects update:

health issue assessment, final report. Washington, DC: US Environmental Protection Agency, Office of Health and Environmental Assessment, 1984:2.1-2.9. *This concentration is based on an assumption that the lost mercury had completely vaporized and had thoroughly mixed with the air in the room. **Body burden was estimated using the value of the mercury saturation point in air and assuming 100% absorption of mercury in the lungs and a breathing rate of 20 m3 per 24 hours for a period of 3/4 of an hour.

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